Re: Slowdown in assignment mystery

[David Bateman]

John Swensen wrote:
> I have a fairly simple example that has me perplexed.  Below are the two 
> code listings.  As a little background, the ocw_grab_frame() function 
> returns a uint8 matrix.  Also, this computer has 4GB of RAM, and we are 
> running this from a Linux box with X11 *not* running so less than 300MB 
> is in use before running octave and the swap hasn't been touched.  I 
> would expect the first listing to run the fastest.  Since we are 
> pre-allocating the space for the images, then it seems like it should be 
> a simple memcpy into the new buffer.  Contrary to this assumption, the 
> first listing has a mean(tocs)=200ms and std(tocs)=10ms, where the 
> second listing has a mean(tocs)=11.2ms and std(tocs)=1.5ms.  I am a 
> little confused, so maybe someone has a suggestion.
>
> John Swensen
>
> Listing 1:
> imgs = uint8(zeros(480,640,100));
> for I=1:100
>     tic;
>     imgs(:,:,I) = ocw_grab_frame();
>     tocs(I) = toc;
> end
>
> Listing 2:
> for I=1:100
>     tic;
>     img = ocw_grab_frame();
>     tocs(I) = toc;
> end
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>
>   
Have you tried without the tic/toc's at all and just use wall-time?
Maybe tic/toc itself is the issue..

D.

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